1. Field of the Invention
The present invention relates to a cathode for use in a molten carbonate fuel cell (MCFC) and methods for preparation thereof. More particularly, the invention relates to a LiCoO.sub.2 -coated NiO cathode for use in a molten carbonate fuel cell and methods for preparation thereof, which comprises coating LiCoO.sub.2 on the surface of a NiO cathode for MCFC to reduce dissolution of NiO into electrolytes. The present invention provides a cathode for a molten carbonate fuel cell having a long lifetime as compared to the prior cathode while maintaining the performance of the fuel cell.
2. Description of the Background
In the prior art, nickel oxide (NiO) which has been usually used as a raw material in the cathode for a MCFC is known as an optimal cathode material because it is cheap and its electrochemical performance excellent . However, when cell operation times become longer, NiO is liable to dissolve into electrolytes as Ni ions and the dissolved Ni ions may be precipitated in the electrolytes matrix to cause electrical shorting of the cell, which shorten the lifetime of the cell.
Therefore, many attempts have been made to reduce dissolution of NiO into the electrolytes.
For example, a method is proposed for inhibiting the dissolution of the electrode by controlling the operation condition of the cell. Generally, dissolution of NiO on the electrolytes may depend upon partial pressure of CO.sub.2 in the cathode gas, cell operation temperature, concentration of H.sub.2 O, etc. Reduction of the dissolution of the NiO through the control of operation conditions may increase the lifetime of the cell though it cause to slightly reduce the cell performance. For example, the partial pressure of the CO.sub.2 in the cathode is lowered and the thickness of the matrix is increased, and then it takes the longer time to reach to shorting of the cell so that the lifetime of the cell may be extended (See, A. J. Appleby and F. R. Foulkes, "Fuel Cell Handbook", 570, Van Nostrand Reinhold, New York (1988)). However, the above approach results in the problem in that the cell performance is reduced and that the extension of the lifetime of the cell is not so long.
Also, other attempts have proposed to control the components of carbonate used as electrolytes in the MCFC or to add additional components such as a basic component to NiO electrode. Since NiO is usually dissolved into the electrolytes by acidic dissolution mechanisms under the general cell operation condition of MCFC, increase of the basicity of the electrolytes cause to lower the dissolution of the NiO. For examples, instead of eutectic salts comprising of 62 mole % of Li.sub.2 CO.sub.3 and 38 mole % of K.sub.2 CO.sub.3 which are usually used as electrolytes in the prior art, eutectic salts containing the high contents of Li.sub.2 CO.sub.3 or Li.sub.2 CO.sub.3 -Na.sub.2 CO.sub.3 eutatctic salts may be used. There have also been attempts to propose to add carbonate of alkali earth metal such as MgCO.sub.3, CaCO.sub.3, SrCO.sub.3, BaCO.sub.3 into the electrolytes comprising of 62 mole % of Li.sub.2 CO.sub.3 and 38 mole % of K.sub.2 CO.sub.3, or to add alkali earth metal oxide such as MgO to a NiO electrode itself. See, J. D. Doyon, T. Gilbert, G. Davis, J. Electrochem. Soc., 134, 3035-3038 (1987); K. Tanimoto et al., J. of Power Sources, 39, 285-297 (1992); K. Ota, Proceedings of the Fourth Internal Symposium on Carbonate Fuel Cell Technology, Ed. by J. R. Selman, The Electrochemical Soc., Pennington, N.J., 238-252 (1997); and H. J. et al., J. of Power Sources, 61, 239-245 (1996). However, when composition of the electrolytes is varied so as to increase their basicity, such a change of the composition may result in adverse effects on the cell performance.
Furthermore, there have been attempts to propose to develop substitute materials for NiO in the fuel cell. Up to now, lithium compounds such as LiFeO.sub.2, LiMnO.sub.2, LiCoO.sub.2, etc., have been discussed as such substitute materials. However, LiFeO.sub.2 and LiMnO.sub.2 have poor cell performance. LiCoO.sub.2 is known as a reliable substitute material, but it has lower electrical conductivity than that of NiO. Thus, resulting in poor cell performance. Also, the mechanical strength of an electrode made from LiCoO.sub.2 is weak resulting in difficult installing electrodes with fuel cells. In addition, the price of Co precursor is relatively expensive, increasing production costs of the cell. See, Plomp, J. N. J. Veldhuis, et al., J. of Power Sources, 39, 369-373 (1992); C. Lagergren, et al., J. Electrochem. Soc., 141, 2959 (1994). in cases that where such substitute materials are used as electrodes in fuel cells, although the dissolution of the cathode on the electrolytes is reduced by 1/10 of conventional cathode, there arises many problems, such as deterioration of the cell performance due to a reduction in electrical conductivity, difficulty in producing of the large size cells due to the poor mechanical strength of the electrode, expensive production costs, etc.
The present inventors have carried out many investigations in order to develop a cathode for use in MCFC having long lifetime without the above problems. As a result, it has been found that a cathode having a long lifetime while maintaining its cell performance can be obtained by coating LiCoO.sub.2 on the surface of the NiO cathode to stabilize NiO.